Due to the improvement in various technologies, the volume of information transfer required by human life, including fine pictures, multimedia broadcasts and human-computer mutual action is increasingly huge, therefore a technology to provide more high speed data transfer has thus been developed. With regards to communication systems, there is a direct correlation between the speed of data transfer and the size of the bandwidth of the system. Under same time limitation, a system with a bigger bandwidth would be able to transfer a bigger data volume. Furthermore, with an increasing focus on mobility and simplicity of design, the use of wireless systems to replace wire-based systems in communication and data transfer has already become widespread.
In a well-known communication system, a circuit to manage radio frequency signals generally comprises a transmitter and a receiver. As shown in FIG. 1, the radio frequency signals received via an antenna (01) enters into a receiver (02), then the signals is amplified by a low noise amplifier (03) and frequency of the amplified signals is transformed to baseband by a step-down converter (04); after that, the baseband signals are converted from an analog form to a digital form via an analog-to-digital converter (05), followed by carrying out digital signal processing (06), and finally the signals are sent to the application end (10). The reverse path of the signal is: the digital baseband signals from the application end (10) are passed through digital signal processing (06), and then the signals are converted from a digital form to an analog form via a digital-to-analog converter (05′), and then passed through an upconverter (07) and a power amplifier (08) and converted to radio frequency signals with a proper frequency. The downconverter (04) and upconverter (07) carries out a downconversion in frequency or a upconversion in frequency of the transmission or reception signals based on the broadcast signals from a local oscillator (09), and the low noise amplifier (03) at the receiver (02) is a crucial circuit that affects the performance of the entire system. With regards to a broadband wireless communication system, the low noise amplifier (03) must be within the required frequency range, and simultaneously it must meet the requirement of having a good broadband input impedance matching characteristic, low noise performance and a sufficiently high gain and in-band gain flatness. However, with respect to current technology, it is contradictory to design a system simultaneously capable of providing with broadband input impedance matching and broadband low noise performance.
The circuit of a conventional low noise amplifier is primarily based on the theory of resonance generated from a single group of inductance capacitance, and suitable for narrowband application of which the fractional bandwidth is less than one percent can be used. In the paper of D. K Schaeffer and T. H. Lee, “A 1.5V, 1.5 GHz CMOS low noise amplifier” (IEEE J. Solid-State Circuits, Vol. 32, No. 5, May 1997, p. 745 to 759), it is disclosed that among the various narrowband low noise amplifiers, as shown in FIG. 9, an inductive source degenerative common-source amplifier has the best performance level in terms of low noise and low power consumption, and can provide good impedance matching and signal amplification within a narrowband range.
In order to extend conventional narrowband circuits to the broadband domain, in the paper of A. Bevilacqua and A. M. Niknejad, “An Ultra wideband CMOS Low-Noise Amplifier for 3.1-10.6 GHz Wireless Receivers” (IEEE J. Solid-State Circuits, Vol. 39, No. 12, p. 2259 to 2268, Dec. 2004), and the article “A 3 to 10 GHz Low Noise Amplifier with Wideband LC-Ladder Matching Network” (IEEE J. Solid-State Circuits, Vol. 39, No. 12, pp. 2269 to 2277, December 2004) by A. Ismail and A. A. Abidi, both disclose the possibility of adding a multi-order band-pass filtering circuit to an inductive source degenerative common-source amplifying transistor, as shown in FIG. 10. Although the technology is able to provide broadband input impedance matching, the low noise performance is still kept in the narrowband range. Furthermore, too many inductors and capacitors will cause the area of the circuit board to be exceptionally large.
In the article of Robert Hu and M. S. C. Yang, “Investigation of Different Input-Matching Mechanisms used in Wide-Band LNA Design” (International Journal of Infrared and Millimeter Waves, vol. 26, no. 2, pp. 221 to 245, February 2005), it proposes the use of an inductive source degenerative source amplifier and a capacitive feedback amplifier simultaneously, as shown in FIG. 11. Although in theory the circuit is able to provide a better broadband impedance matching, but the broadband performance is significantly reduced as a result of the parasitic resistance in a transistor.
With respect to the application of transformer feedback, in the article of D. J. Cassan and J. R. Long, “A 1-V transformer-feedback low noise amplifier for 5 GHz wireless LAN in 0.18 um CMOS” (IEEE J. Solid-State Circuits, Vol. 38, No, 3, March 2003 pp. 427 to 435) and U.S. Pat. No. 6,026,296 A, have mentioned about the research and results with regards to the use of a transformer on a low noise amplifier are documented. As shown in FIG. 12, the circuit design is based on the principle of uni-lateralization, and consequently the design is only applicable in narrowband improvements, and cannot be applied for broadband frequencies.
Summing up, the objective of those researches concerning radio frequency circuits industry is to develop a circuit structure for a broadband low noise amplifier, which could break the limits of narrowband and achieve an optimal broadband flat gain, broadband impedance matching and broadband low noise performance.